The issue is that no matter which way you go, floating rates depending on online players are a bad idea.
If you go higher rates for less players, people will only want to log on with low pop to get the bonus, thus never reaching a high pop (which is what we want).
If you go higher rates for more players, you get the weird paradox of people only wanting to log on when it's high pop to get the bonus but not reaching high pop because everyone has the same mindset.
Personally I don't think we need floating rates at all, but if people really do want it, there are better ways of doing it (e.g. unlocking a 24h floating rates event through zeny donated by everyone on the server, thus also creating a zeny sink).

There's a version of floating rates activated by players for like a day, through paying zeny to the npc - which would both be a zeny sink and unpredictable (hence not killing parties when it's not on).

I don't think #help should have a delay, since actual conversations with follow ups happen there, whereas #trade and #recruit is just putting your message out there and waiting for whispers in response.
I'm also pretty sure a zeny cost 1. wouldn't stop the spam and 2. would probably be hard to code, whereas the delay is already part of the channel system.

Idea description:
As the title says; a lot of these add from quests, and unless you do the quest again or specifically go back to the NPC to get rid of the cooldown quest, it will stay in your quest window forever. (not quite on topic, but getting rid of leftover job change quest bits would be great too, like "travel to the archer's town" etc, since you can't delete those at all, just hide them)
Why is it good?
Less clutter, mostly.
How urgent is this rate 1-10:
2
Will it affect our economy?
No
Will it affect any of our core systems?
No
If we deny it, what might happen?
More quest window clutter
If we accept it, who does it benefit the most?
Anyone doing quests
You promise that this idea is for the better of the server in whole:
Yup
Other inputs:
I like my quest window tidy :v